Document J30RYKknkmxbrq70B4MLY66dv

820 CHAPTER 36 2$: 1954fQiudk J A " 9.78 X 573 - 102 = g23. ' O <70 (5) The total coil loadmay be calculated from the enthalpy difference across the coil and the air quantity using the weight of dry air instead of the weightof the mixture. .,wi , 3, = (7. (fti.f- hi), = 1700 (49.24 -- 23.77), ... = 43,200 Btu per (hr) (sq ft of face area). where (?,, = weight of dry air per (hour) (square foot of coil face area), hi = enthalpy of air vapor mixture entering coil, Btu per pound of dry air. hi = enthalpy of air-vapor mixture leaving coil, Btu per pound of dry air. Fig. 14 Psychbometbic Layout FOK. CoiL .SELECTibN tfsiNG. REHBAT (6) The refrigerant temperature may be found from Equation 9 43,200 = . - t,) .=22.1 325- 15 X 6 X 15 Therefore,'ir = (52.3 - 22.1) = 30.2. Thus a coil 6 rows deep, ojierating at a refrigerant temperature of 30.2 ^ and a .face velocity of 400 fpm', is required; and it will carry a total load of'43;200 Btu`per (hour) (square foot of face area). The air conditions leaving the coil are too low for the con. ditions of the problem and therefore it is necessary to by-pass air at the entering condition to obtain the desired result of 80.5 F dry-bulb and 73 F wet-bulb. csaiirsseAAeedsslltthhlttooooogauuduuggsshhreeattaathhioeehh.iippggrrhheeSeeccurreecddrhreeiinnaffrrggiiggsmsoeeolrrwulaautnnti`oijtotnjntteeijimmsso ppss..heea..orr.t.aaiw.s.tt_fu~una_rr_ceein_to_aa'Frcnn,yi_idd:,g-_i.ee_t_mm1mA4pp.allooyTyyItbnerreett.mhhhMeeionsaarecttcaattdsooseees,oo,itrbbthahttebaaeliiennccoiottnihnilleeslelooadwma*ede ratio line intersects the saturation curve and, therefore, a coil of any depth may be . atio lin selleeIcftaedc.oil depth of 6 rows is maintained, the exi,t air conditions for the coil are indi- Air Heating and Cooling Coils 821 cated at point. B, I?ig.,,14 as .72.3 F dryTbulb and 70.8 F wet-bulb,, and; the.surface temperature will be ' ,.= 9.78 X 72.3 - 102 " 8.78 69.0. The coil load willbe: gt ;= 1700,(49.24 -- 34.66) = 24,800.Btu per (hour) (square foot of face area) and the refirjgera.nt temperature. will.be found drum,Equation 9: . 24,800 .................... '"'325: W`(A- tr) 12.7 1S ,X 6 X Te ' Therefore, t, = 69.0--T2.7 = 56.3. ., . _. Thus, for the case where reheat is used, .a coil 6 rows deep.operating.at a refrigerant temperature of 56:3 F is required'. The total coil load will be 24,800. Btu. per (hour) (square foot of face area) but the actual effective load will be less by the amount of reheat required.' Therefore','for a given load, a larger coil and'more refrigerating capacity are required when reheat is used. ... ' LETTER SYMBOL? USED IN CHAPTER 36 . i) = fin efficiency. ` A = external area of;coil,. square feet per (square foot of: coil.face area), (row . of coil depth). jg __ tl. --' tdpl . .. . U. -- taps- :v''' 1:1 '. D = internal diameter of tube, inches. '. V. ; ... G = air mass velocity, pounds per ;(hour) (square1foot of coil face area). = dry air mass'velocity, pounds dry air per (hour),'(Square foot of coil face; area). /i = film coefficient of heat transfer between fluid and internal coil surface, Btu per (hour) (square foot internal surface) (Fahrenheit degree mean temperature between fluid and surface). fo = film coefficient of heat transfer between air and external coil surface, ' Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference between air and coil). - - -- hi = enthalpy of air-vapor mixture entering coil, Btu per pound of dry air. hi = enthalpy of air-vapor mixture leaving coil, Btu per pound of dry air. h = conductivity of pipe or tube material, Btu (square foot) (hour) (Fahren heit degree per inch thickness). L = thickness of tube wall, inches. N = number of rows of coil depth. 0 = a constant, exponent of G in Equation 8, obtained by plotting, on loga rithmic coordinates, G against values of The value of n is the slope of the line. ? = sensible heat transferred, Btu per (hour) (square foot of coil face area). 3t = total heat transferred by coil, Btu per (hour) (square foot of face area). = ratio between external and internal surface of tube. * = average water temperature, Fahrenheit degrees, b = dry-bulb temperature of air entering coil, Fahrenheit degrees, b = dry-bulb temperature of air leaving coil, Fahrenheit degrees.